1. Field of the Invention
This method relates to a method of improving the image quality in magnetic resonance images (MRI) when the timing of image acquisition is synchronized to a physiological cycle or interval. More particularly, there is a variation in the length of each cardiac cycle which produces a reduction in image quality in the final image. By measuring the cycle length associated with each data acquisition, the present processing method uses this information to correct the data. This results in improved image quality permitting improved visualization of anatomical structures.
2. Description of the Prior Art
Cardiac synchronized image acquisition is a standard method required in magnetic resonance imaging (MRI) to produce a clear image of the heart at specific phases of its cycle in order to assess the structure and/or function of the ventricles, atria, coronary arteries or great vessels. In addition, it is widely used to reduce blood flow artifacts in spine and abdominal imaging.
Uneven data sampling intervals (TR) between MRI data acquisitions exist in cardiac synchronized studies due to heart rate variability and produce artifacts indistinguishable from those caused by object motion. These are seen as smearing or object replication (ghosting) along one direction of the image matrix. Because the time between heart beats is not sufficiently long relative to the T1 relaxation constant of most tissue, the return of the magnetization vector to its equilibrium position (longitudinal recovery) is different for each data sampling. This results in a modulation of signal intensity along the phase direction in the pre-Fourier transformed magnitude data in turn produces reduced quality final images. A method to eliminate this form of image artifact would enhance the diagnostic value of heart cycle synchronized magnetic resonance images, but is not taught by the prior art.
U.S. Pat. Nos. 5,159,550 and 5,138,259 describe methods for MRI artifact suppression. However these patents address artifacts caused by object motion through the imaging plane and that of inhomogeneities in the static magnetic field respectively. Neither are directed toward the artifacts specific to heart cycle synchronized MRI, nor is the length of any physiological cycle used in the method of correction.
The following articles discuss various causes of MRI artifacts and present previous methods for reducing their negative impact on image quality;
1. "Respiratory effects in two-dimensional Fourier transform MR imaging" by L. Axel, R. M. Summers, H. Y. Kressel and C. Charles (Radiology vol. 160, p795-801, (1986)) PA0 2. "Respiratory ordered phase encoding (ROPE): a method for reducing respiratory motion artefacts in MR imaging" by D. R. Bailes, D. J. Gilderdale, G. M. Bydder, A. G. Collins, and D. N. Firmin (J. Comput. Assist. Tomogr. vol.9, p835-838 (1985)) PA0 3. "Practical aspects of ghosting in resistive nuclear magnetic resonance imaging systems" by R. A. Lerski, K. Straughan and J. L. Williams (Phys. Med. Biol. vol. 31, p721-735 (1986)) PA0 4. "The effect of motion on two-dimensional Fourier transformation magnetic resonance imaging" by C. L. Schultz, R. J. Alfidi, D. Nelson, S. Y. Kopiwoda and M. E. Clampitt (Radiology vol. 152, p117-121 (1984)) PA0 1. Simultaneous recording of the length of the heart or other appropriate cycle length associated with MRI data acquisition at each phase encoding position. PA0 2. Normalization of the signal intensity for each phase encoding position in the image matrix to that which would have existed in the presence of a fixed cycle length as follows: EQU Signal correction factor=Signal.sub.mean /Signal.sub.individual( 2) PA0 3. After the acquired data has been corrected for the effect of cycle length variations, a standard Fourier transformation is applied along the X and Y image matrix dimensions to generate the spatial domain image that is used for diagnosis.
The methods disclosed in these articles do not teach how to make use of the physiological cycle lengths associated with an image to correct the data and improve the image quality. Similarly, an article entitled "Practical aspects of ghosting in resistive nuclear magnetic resonance imaging systems" by R. A. Lerski, K. Williams, and J. L. Williams (vol.31 Phys. Med. Biol. (1986)) teaches that a modulation of the signal amplitude in the phase encoding direction (caused by electronics not physiology), will result in the production of image "ghosts" but does not disclose a method of data correction to overcome such a problem. Additionally, in chapter 3, page 25 and chapter 11, page 109 of the text "Quality assurance and image artifacts in magnetic resonance imaging" published by Little, Brown and Company, R. James Knowles and John Markisz teach that gating or synchronization of rf pulses (used in image formation) produce variable recovery times and thus modulate the T1 contrast of the image. They further teach that arrhythmias such as PVCs or changes in heart rate during the scan can reduce image quality. In neither case do they disclose the underlying effect of heart cycle variation on artifact formation nor disclose a correction.